National Semiconductor MM58274C overview
National Semiconductor MM58274C Real-Time Clock When a Real-Time Clock (RTC) is to be added into a digital system, the designer will face a number of design con...
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Manufactured byNational Semiconductor
Product groupReal-Time Clocks
DescriptionNational Semiconductor MM58274C Real-Time Clock
OverviewWhen a Real-Time Clock (RTC) is to be added into a digital system, the designer will face a number of design constraints and problems that do not usually occur in normal systems. Attention to detail in both hardware and software design is necessary to ensure that a reliable and trouble free product is implemented.The extra circuitry required for an RTC falls into three main groups: a precise oscillator to control real-time couting; a backup power source to maintain time-keeping when the main system power is removed; power failure detection and write protection circuitry. The MM58274C in common with most RTC devices uses an on-chip oscillator circuit and an external watch crystal (frequency 32.768 kHz) as the time reference. A battery is the usual source of backup power, along with circuitry to isolate the battery-backed clock from the rest of the system. Like any CMOS component, the RTC must be protected against data corruption when the main system power fails; a problem that is very often not fully appreciated.Rather than dealing strictly with any one particular application, this applications note discusses all of the aspects involved in adding a reliable RTC function to a microprocessor system, with descriptions of suitable circuitry to achieve this. Hardware problems, component selection, and physical board layout are examined. The software examples given in the data sheet are explained and clarified, and some other software suggestions are presented. Finally a number of otherwise unrelated topics are lumped together under "Miscellany"; including a discussion on how the MM58274C may be used directly to upgrade an existing MM58174A installation.
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